Conventionally in a car, there is adopted a communication system, to be mounted on a car, in which messages are sent and received among electronic control units (ECU) by constructing a network in which a plurality of terminals is connected to a common bus. In the network, ECUs controlling the operation of appliances mounted on the car are connected with each other via a bus for multiplex communication.
CAN (Controller Area Network) is widely used as the communication protocol of the communication system to be mounted on a car. The CAN is an asynchronous communication allowing any of the ECUs to send messages when the bus is in an open state.
When messages are simultaneously sent from a plurality of the ECUs, arbitration is executed to allow the transmission of a message having a higher priority to be effective. Therefore there is a case in which a certain amount of time elapses before the ECU having a low priority sends a message.
Therefore in designing the message communication among the ECUs, there occurs a problem that in supposition of the longest delay period of time caused by a defeat in the execution of the arbitration, a load factor to be applied to the bus cannot but be set low.
In the communication system to be mounted on a car, there is a demand for making transmission timings as close as possible in each group by dividing messages into some groups.
Messages are so divided that messages relating to each other fall into the same group. For example, messages relating to a steering system fall into the same group. Upon receipt of a message, sent from one of other ECUs, which belongs to the same group, one ECU sends a message in response. At this time, the transmission timings are made as close as possible in each group, whereas in a different group, transmission timings are made as distant as possible. Thereby delay in response decreases, which allows electric appliances relating to the steering system and the like to be controlled rapidly.
The present applicant proposed the control system for controlling the transmission timings of messages, as described in Japanese Patent Application Laid-Open No. 2007-60400 (patent document 1). In the control system, each of a plurality of the ECUs connected to the bus has the time measuring means. Each ECU measures the transmission timings of messages sent from other ECUs and adjusts the transmission start timings of messages to be sent from each ECU according to values indicated by the time measuring means. Owing to the adjustment, the ECU makes the transmission timings of messages to be sent therefrom later than the transmission finish timings of messages the other ECUs have sent. Thereby collision between the messages sent from other ECUs is prevented.
The method of the patent document 1 is effective in determining the transmission timings of messages to be sent from all ECUs on the basis of one ECU. But the communication system to be mounted on a car has become large, complicated, and many kinds of the communication system to be mounted on a car is in production. When the reference ECU which may be called a time prototype is set, influence is great when the reference ECU has failure. Thus to determine the transmission timing in dependence on the reference ECU has a problem. Considering a case in which the reference ECU is different in dependence on the kind of a car, the method of the patent document 1 tends to increase the kind of the ECU, and there is a possibility that the degree of freedom of mounting the kind of the ECU is impaired.
Even when there is no reference ECU, the method of the patent document 1 is applicable to the communication system to be mounted on a car. But in this case, there is a possibility that the transmission timings of messages ECUs have sent do not become close to each other and fall into an oscillation state. This is shown by using an example of FIG. 13.
For example, in a case where it is desirable for ECUs to send messages at timings as close as possible, an ECU A, an ECU B, and an ECU C are considered. Let it be supposed that an average period of the entire communication system is about 9 ms (msec). Let it be also supposed that of the transmission periods shown in FIG. 13(A), the transmission time of a first period is set as an initial condition and that in periods subsequent to the second period, transmission timings are so set that the ECU A sends a message at the transmission timing of the ECU B of the first period, the ECU B sends a message at the transmission timing of the ECU C of the first period, and the ECU C sends a message at the transmission timing of the ECU A of the first period. At this time, numerical values shown in FIG. 13(A) indicate the transmission order. The transmission timings of the ECUs remain oscillated and do not become close to each other nor converge and in addition even skip-caused missing of messages occurs in the transmission thereof.
Even though the transmission timing is so set that one ECU sends a message to one of other ECUs and thereafter sends a next message thereto after the other ECU sends a message at an initial transmission timing+9 ms, the transmission order is as shown in FIG. 13(B). Similarly to the transmission timing shown in FIG. 13(A), the transmission timings of the messages sent from each of the ECUs remain oscillated and do not become close to each other nor converge and in addition even skip-caused missing of messages occurs in the transmission thereof. Similarly even though an average value of transmission timings of other ECUs is used, failure occurs.
That is, when the reference ECU is not set, there is a case in which the application of the method of the patent document 1 does not allow messages each of the ECUs has sent to become close to each other and to converge. Thus there is room for improvement in the method of the patent document 1.
A method of making transmission timings of messages distant from each other when the messages can be divided into not less than two groups is not disclosed in the patent document 1. A method of outputting messages by spacing transmission timings from each other at constant time lags is described in the patent document 1. This method is incapable of making the transmission timings of two groups distant from each other unless the average period of the entire communication system is known. In addition, in dependence on an operation state, the average period of the entire communication system of a car changes. Thus the method of the patent document 1 is incapable of making the transmission timings of the two groups distant from each other. Therefore there is room for improvement in the method of the patent document 1.
Patent document: Japanese Patent Application Laid-Open No. 2007-60400